1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive composition required to be transparent for an optical film, a pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition for an optical film and a production method thereof. The present invention also relates to a pressure-sensitive adhesive optical film including an optical film and the pressure-sensitive adhesive layer formed on at least one side of the optical film. The present invention further relates to an image display such as a liquid crystal display and an organic electroluminescence (EL) display, including the pressure-sensitive adhesive optical film. The optical film may be a polarizing plate, a retardation plate, an optical compensation film, a brightness enhancement film, a laminate thereof, or the like.
2. Description of the Related Art
The image-forming system of liquid crystal displays or the like requires polarizing elements to be placed on both sides of a liquid crystal cell, and generally polarizing plates are attached thereto. Besides polarizing plates, a variety of optical elements have been used for liquid crystal panels to improve display quality. For example, there are used retardation plates for prevention of discoloration, viewing angle expansion films for improvement of the viewing angle of liquid crystal displays, and brightness enhancement films for enhancement of the contrast of displays. These films are generically called optical films.
When the optical films are attached to a liquid crystal cell, pressure-sensitive adhesives are generally used. Bonding between an optical film and a liquid crystal cell or between optical films is generally performed with a pressure-sensitive adhesive in order to reduce optical loss. In such a case, a pressure-sensitive adhesive optical film including an optical film and a pressure-sensitive adhesive layer previously formed on one side of the optical film is generally used, because it has some advantages such as no need for a drying process to fix the optical film. In general, a separator (a release film) is also attached to the pressure-sensitive adhesive layer.
The pressure-sensitive adhesive is required to have some characteristics. In some cases, for example, if in the process of bonding an optical film to a liquid crystal cell, they are misaligned or foreign matter is caught on the bonding surface, the optical film should be separated from the liquid crystal panel such that the liquid crystal cell can be recycled, and the pressure-sensitive adhesive should have re-peelability (reworkability) such that the optical film can be easily peeled from the liquid crystal panel with no adhesive residue in a peeling process. Particularly in recent years, thin liquid crystal panels having chemically-etched glass plates are frequently used together with conventional panel manufacturing processes, and it has become difficult to subject optical films from the thin liquid crystal panels to reworking or working processes.
In general, the optical pressure-sensitive adhesive is applied to an optical film to form a pressure-sensitive adhesive layer, and then the optical film is wound into a roll and subjected to a punching process. The optical pressure-sensitive adhesive generally includes an acrylic polymer as a base polymer and a crosslinking agent. For example, the crosslinking agent includes an isocyanate compound. When an isocyanate compound is used, however, it takes a long time to form the pressure-sensitive adhesive layer (for aging), and a long time is required until shipment. If a working process is performed without the aging, fouling such as pressure-sensitive adhesive fouling and dropout fouling will occur to degrade workability. In addition, defects such as dents may be generated during the aging. Therefore, the optical pressure-sensitive adhesive is required to have workability such that it can be worked without pressure-sensitive adhesive fouling or dropout.
It is proposed that peroxide should be used as a crosslinking agent for optical pressure-sensitive adhesives. Crosslinking with peroxide can be completed by a curing process after dying and therefore has the advantage of no need for aging time. When peroxide is used, however, peeling strength for separators can significantly increase so that it can be laborious for panel makers to peel off separators (release films), which can cause some problems such as manufacturing line stoppages. In recent years, as optical films become thinner, peelability or releasability easier than that of conventional separators and better workability are also required.
In addition, the pressure-sensitive adhesive is required not to cause any defect in durability tests by heating, moistening and so on, which are generally performed as accelerated environmental tests.
A conventionally proposed method for solving the problems with the reworkability of liquid crystal panels includes adding a plasticizer or an oligomer component to an acrylic polymer used as a base polymer for an acrylic pressure-sensitive adhesive (see Patent Literature 1 listed below). However, such an acrylic pressure-sensitive adhesive cannot provide satisfactory reworkability for the thin liquid crystal panels as mentioned above, satisfactory processability for pressure-sensitive adhesive optical films or satisfactory workability for separators.
Other proposals include an acrylic pressure-sensitive adhesive including an acrylic polymer to which a peroxide is added (see Patent Literature 2), the use of alkyl (meth)acrylate, a monomer having a hydroxyl group in the molecule and a monomer having a functional group such as a carboxyl, amide or amino group in the molecule as monomer components to form an acrylic polymer (see Patent Literatures 3 and 4), and the use of alkyl (meth)acrylate and a nitrogen-containing monomer such as an imide group-containing monomer or an amide group-containing monomer as monomer components to form an acrylic polymer and the use of a peroxide and an isocyanate compound (see Patent Literature 5). However, none of these acrylic pressure-sensitive adhesives disclosed in the patent literatures can provide satisfactory long-term durability or reworkability, or sufficient processability for pressure-sensitive adhesive optical films or sufficient workability for separators, although they can provide improved initial durability.
Patent Literature 1: JP-A No. 2003-329837
Patent Literature 2: JP-A No. 2006-183022
Patent Literature 3: JP-A No. 2004-091499
Patent Literature 4: JP-A No. 2004-091500
Patent Literature 5: JP-A No. 2007-138147